2. • Atrioventricular Re-entry Tachycardia
(AVRT) is a form of paroxysmal supraventricular
tachycardia that occurs in patients with accessory
pathways, usually due to formation of a re-entry
circuit between the AV node and accessory
pathway
• ECG features depend on the direction of
conduction, which can be
orthodromic or antidromic
4. • In orthodromic AVRT, anterograde conduction is via
the AV node, producing a regular narrow complex
rhythm (in the absence of pre-existing bundle branch
block)
• In antidromic AVRT, anterograde conduction is via the
accessory pathway (AP), producing a regular wide
complex rhythm. This can be difficult to distinguish
from ventricular tachycardia (VT)
• Often triggered by premature atrial or premature
ventricular beats
• In both forms, the features of pre-excitation are lost
• Tachyarrhythmias in pre-excitation can also be
facilitated by direct conduction from the atria to the
ventricles via the AP, bypassing the AV node. This is
seen with atrial fibrillation or atrial flutter in conjunction
with WPW, and is discussed further here
5. • Orthodromic AVRT
• In orthodromic AVRT, anterograde conduction occurs via the
AV node, resulting in a normal direction of ventricular
depolarisation
• This can occur in patients with a concealed pathway (AP that
conducts retrograde only, not evident on sinus rhythm ECG)
• ECG features of AVRT with orthodromic conduction:
Rate usually 200-300 bpm
• Retrograde P waves are usually visible, with a long RP
interval
• QRS < 120ms unless pre-existing bundle branch block, or
rate-related aberrant conduction
• QRS alternans: phasic variation in QRS amplitude associated
with AVNT and AVRT, distinguished from electrical
alternans by a normal QRS amplitude
• Rate-related ischaemia is common
7. • Orthodromic AVRT, or just AVNRT?
• This rhythm can appear very similar to AVNRT, but the RP
interval can assist us to differentiate:
• In typical AVNRT, retrograde P waves occur early, so we
either don’t see them (buried in QRS) or partially see them
(pseudo R’ wave at terminal portion of QRS complex)
• In AVRT, retrograde P waves occur later, with a long RP
interval > 70 msec
• In the above example, look closely at V1 — P waves are
evident as a small notch at the beginning of the T wave, with a
long RP interval, indicating this is likely orthodromic AVRT
• Fortunately, treatment is fairly similar for both
8. • Treatment of orthodromic AVRT
• As always, patients that are unstable due to this rhythm
require urgent DC cardioversion
• The anterograde portion of conduction is typically the “weak
link” of the re-entry circuit. Management options in the stable
patient therefore target slowing conduction through the AV
node
• A stepwise approach similar to AVNRT can be employed,
beginning with vagal manoeuvres followed by adenosine
and/or verapamil
• Note that with administration of any AV nodal blocking drug,
there is a very small but significant risk of inducing AF. If
verapamil is used, patients should be observed for at least 4
hours to ensure AF does not develop as a consequence of AV
nodal blockade
9. • Antidromic AVRT
• Antidromic AVRT is rare, and makes up only 5% of
tachyarrhythmias in patients with WPW
• As the name suggests, it involves anterograde
conduction via the AP
• Retrograde conduction is usually via the AV node,
but can also be via another AP
• The abnormal direction of ventricular depolarisation
results in a broad complex tachycardia, which can
be easily mistaken for VT
• ECG features of AVRT with antidromic
conduction:
• Rate usually 200-300 bpm
• Wide QRS complexes due to abnormal ventricular
depolarisation via AP
11. • Treatment of antidromic AVRT
• This rhythm can be difficult to distinguish from VT,
and if there is any doubt, we should presume a
diagnosis of VT and treat accordingly
• In stable patients, drug therapy should be targeted
at the AP
• Procainamide (class I) would be our first line
antiarrhythmic. Ibutilide (class III) and amiodarone
are second-line options, but their effectiveness is
less established
• DC cardioversion may still be required if drug
therapy fails
